Which types of insulating oil can be tested?
While the generic term "oil" is almost universally used to describe insulating fluids, there are currently five different types of insulating fluid in common use. These are:
(1) Mineral oil
(2) High molecular weight hydrocarbon (HMWH) fluids
(3) Silicone fluids
(4) Synthetic ester fluids
(5) Natural ester (vegetable oil) fluids
All of these oil types can be tested for dielectric breakdown voltage.
Mineral oil is the most common insulating fluid and has been in use since the late 19th century. There are many mineral oil filled transformers that have been in continuous use for more than 50 years. Mineral oils are refined from either naphthenic crude or more recently, from paraffinic crude.
HWMH, silicon, synthetic ester and natural ester fluids are more recent developments and are often preferred because they are much less flammable than mineral oil. ASTM D5222 specifies that for insulating fluids to qualify as "less flammable" they must have a fire point of at least 300℃.
The five fluids differ significantly in the way they behave in the presence of moisture. Mineral oil is the least satisfactory, and even small amounts of water significantly reduce its breakdown voltage. Silicone fluid is also quickly affected by small amounts of moisture, whereas ester fluids behave very well in the presence of moisture and can typically maintain a breakdown voltage of greater than 30 kV with more than 400 ppm water content. This is one of the reasons that esters last much longer in service.
